1. Field of the Invention
The present invention relates to a heat-sealable, opaque, biaxially oriented, multilayer polypropylene film comprising a base layer, and at least one interlayer arranged thereon, and at least one outer layer arranged on the interlayer.
The invention further relates to a process for the production of the multilayer film and to the use of the multilayer film.
2. Description of Related Art
Opaque films are generally known in the art. EP-A-0 180 087 describes a five-layer heat-sealable film which has a vacuole-containing base layer produced from propylene homopolymer and calcium carbonate, an interlayer of propylene homopolymer and hydrocarbon resin, and heat-sealable outer layers comprising propylene-ethylene copolymers.
The thick interlayer of polypropylene and hydrocarbon resin means that the film has better mechanical properties and higher sheen than films of the known art. In addition, the film is readily corona-treated due to the embossed heat-sealing layers of propylene-ethylene copolymers. The film has unsatisfactory sheen, whiteness and opacity.
EP-A-0 312 226 discloses an opaque multilayer film having one outer layer which comprises a readily printable polymer, an interlayer of propylene homopolymer and a base layer containing vacuoles. The film is distinguished by high sheen as seen from the homopolymer side. Due to the disclosed layer thickness ratios, the surface sheen, the opacity and the whiteness are unsatisfactory, as is the case with EP-A-0 180 087.
EP-A-0 408 971 describes a film of high whiteness and high opacity which is readily printable and is said to have sheen. However, the sheen is unsatisfactory due to the large thickness of the outer layer. The film can have a cloudy structure, probably due to the small particle size of the calcium carbonate used. The particle size of 1.0 .mu.m given as a preferred size results in poor dispersibility in the polymer. In addition, the increased fines content of the particles results in reduced opacity, since no vacuoles are formed.
All the known opaque films mentioned above particularly, have the disadvantage of poor antistatic properties and very poor destackability. In addition to their conventional use as packaging films, opaque films are also used as label materials. To be used as labels, the film is cut into pieces of suitable size and stacked one on top of the other. In the labeling process, the individual film pieces are removed from this stack by machine and fed to the labeling process. In the case of conventional opaque films, problems frequently occur in this machine destacking operation if the machine frequently grasps more than one sheet. As discussed above, conventional opaque films have very poor antistatic properties, which results in contamination of the surface by accumulation of dust, and also represents a potential hazard in the case of extreme charging, due to the large surface area of the film. For example, due to sudden discharging in a printing machine, a dangerous situation such as an explosion of solvents could occur.
The known finishing of the base layer with antistatics does not improve the antistatic properties of the film to a sufficient extent. The incorporation of corresponding additives into the outer layer results in production problems due to evaporation. The antistatics deposit in the tenter frame and on the rolls and result in interfering contaminants. In addition, the finally produced film does not contain the intended amount of antistatics.
In addition, known films have unsatisfactory antistatic properties after printing. The films are usually printed over the entire area with a base color, generally white, before application of a colored print image. It has been found that this full-area printing of the film results in a considerable impairment of the antistatic properties, particularly in the case of white full-area printing.